The Dangers of Earthing an
Already-Charged object
This article sets out to dispel the myths surrounding “safe” ways to earth an object
after it has become inadvertently charged with static electricity in a potentially
flammable atmosphere. It draws from a paper given at Explorisk 2002 by M. Glor and
K. Schwenzfeuer of the Swiss Institute for the Promotion of Safety and Security
(www.swissi.ch), along with research carried out at Newson Gale Ltd.
It is generally understood that if a
conductive object, such as a drum, is
earthed with a suitable clamp and cable,
before it is used in an operation, then it can
never become charged with static
electricity. However if the earth connection
was forgotten and the object was allowed to
become electro-statically charged, then
approaching it with an earthed item, such
as a grounding clamp, will cause a spark to
be drawn from it, just before the earth
contact is made. This spark discharge may
cause the ignition of flammable gases,
vapours or even some dusts, if they are
present in optimum concentrations in the
surrounding atmosphere.
The above is true, but it is not the full picture.
A number of devices or methods are in use
which rely on the fact that the grounding
clamp is not directly in contact with earth, in
order to prevent discharges. The two most
common types are:
The Open Earth Cable
Here the earth connection is broken by an
isolation switch, usually in a flameproof
enclosure built into the clamp or situated on
the wall close by. The concept is that the
clamp will not draw a spark from an object
that may have become charged, as the
clamp is not earthed until after it has been
applied and the isolation switch has been
turned to make the earth connection. It is
said that any sparking will occur in the Exd
enclosure.
This is a myth! A spark will not only occur
between a charged object and an earthed
clamp, but between a charged object and
anything with enough electrical
capacitance and at a lower electrical
potential. Research has proved that, in
practice, the clamp and cable length back
to the switch has as much capacitance as a
typical object to be earthed - possibly as
much as a road tanker!
Hence, when the clamp comes close to the
charged object, the capacitance of the
clamp and cable will allow a spark to jump
from the object, regardless of whether the
isolation switch has broken the earth
connection or not. In practice, an even
greater problem can occur with such a
device: after connecting the clamp, the
operator may forget to turn the switch,
leaving the drum unearthed as the
operation is commenced. This would allow
the drum to become charged when,
ordinarily, it would not have.
Earth Cable with High Resistance
Here the earthing clamp contains a resistor,
typically of around 1 megohm. The belief is
that the clamp will not draw a spark from an
object that may have become charged, as
the resistance will impede the connection to
earth and prevent it.
This is another false assumption!
Experimentation has clearly shown that a
1 megohm resistance is not able to prevent
incendive sparks. Even raising the
resistance to 100 megohm or above will not
do this and as the resistance is increased,
the clamp and cable's ability to carry out its
intended function of providing a good earth
connection, is diminished. Moreover, an
incendive spark is just as likely to jump
across to the conductive body of a clamp,
owing to the fact that it has it's own electrical
capacitance, when the clamp is brought
into close proximity with the charged plant
item.
Safe Practice
Both of these methods lead the user into a
false sense of security. In truth, it is essential
to ensure that objects are earthed prior to
being used in a static-generating operation.
However, if an object has inadvertently
been allowed to become charged in a
potentially flammable area, the only safe
ways to proceed are as follows:
1. Test the area with a gas detector (without
getting too close to the drum) and if no
flammable atmosphere is present, attach
the clamp.
2. Leave the object for some considerable
length of time, so that the charge can relax
naturally. In practice, this could be several
hours, depending on the situation.
Conclusion To quote Messrs. Glor and Schwenzfeuer “An incendive spark discharge cannot
be prevented by including a high resistance into the earth circuit. Experiments have shown that the
resulting spark discharges are still able to ignite solvent vapours…. Even if the earth cable is first
disconnected from earth, an incendive discharge will appear.”
Hence it can be seen that the best method of earthing moveable objects is to use regular ATEX
approved clamps with continuous cables back to the earthing point. In order to prevent a staticgenerating operation from starting before the earthing clamp has been correctly attached, Earth
Monitoring systems with interlocks to pumps, valves or mixers may be used, as recommended in the
latest CENELEC “Code of Practice for the avoidance of hazards due to static electricity” (50404).
FILLING
CHARGING
CURRENT(I)
PRODUCT FLOWING
INTO DRUM
An example of how an object could become
charged accidentally would be if the filling of a
drum was to occur, thus providing relative
movement between the product and the drum
(and hence, a charging mechanism) when the
drum had not been correctly earthed.
Static Charge on Road Tankers
Finally, there is the widely held myth that a
tanker earthing clamp must have some
mechanism to prevent it drawing a spark
from a charged object, since the tanker will
arrive holding charge which it picked up on
its journey.
In reality, road vehicles have slightly staticdissipative tyres that help to prevent the
build up of charge by allowing the low
levels of static electricity generated during
motion to flow to earth via the road surface.
The American Institute of Chemical
Engineers publication “Avoiding Static
Hazards in Chemical Operations” states
that “there is negligible practical hazard
from charging via road tyres”. In fact, if this
were not the case, simply a person
approaching a “charged” tanker would be
in danger of drawing a considerable spark.
As we have seen, it is not only an earthed
clamp, but anything with sufficient
capacitance and at a lower electrical
potential that can cause this to occur.
Note: whilst the tanker tyres are capable of
dissipating the relatively low levels of static
electricity generated during travelling, they
should not be relied upon to cope with the
high charging mechanism of prolonged
product transfer (often at speed) to or from the
tanker. In order to control this level of static
charging, a low resistance earth connection
should always be used.
This issue of Ear to the Ground was compiled by the Editorial Team of Ellen Paul, Glynn Warren and Graham Tyers, and published by Newson Gale Ltd, Unit 6,
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